SEXUAL REPRODUCTIONS 12-16 



ficulties in solving problems of this type with the rusts have been 

 pointed out. The smuts are better material than the rusts, but the 

 fact that the parasite is a dicaryon and an obligate parasite is a 

 limiting factor. Venturia is the most favorable organism in which 

 the relation of a parasite to its host can be studied. It is an 8- 

 spored ascomycete which produces haploid spores, the haplophase 

 mycelium constituting the parasitic phase on the apple. The hap- 

 lophase grows well on ordinary media and there is no complicating 

 dicaryophase in the life cycle. The ability to act as a parasite must 

 depend on some nutrient which the host produces that is essential 

 to the parasite, or some relation between metabolic rates of host 

 and parasite, and the extensive work on the nutrition of the asco- 

 mycetes suggest that relations of this type may be rather easily 

 worked out. Venturia genetics has been attacked by Keitt and his 

 co-workers. There are many varieties of apples on which this 

 parasite is pathogenic, and the pathogenicity of the different geno- 

 types of Venturia is highly specialized and clearly under genetical 

 control. It is the ideal situation for an analysis of the interplay of 

 host and parasite and the genetical and nutritional interaction of 

 two organisms bound together by this relationship. Venturia is a 

 typical ascomycete, whose life cycle is practically identical with 

 that of Neurospora, except that the haplophase is pathogenic to 

 various apples. However, the genes controlling pathogenicity seg- 

 regate regularly in the ascus and precise ratios are obtained. 

 In the analysis of a selected stock Keitt has shown that one 

 gene controls parasitism on Haralson and Wealthy apples, while 

 cultures carrying its allele are not pathogenic against these varie- 

 ties, but are pathogenic for Mcintosh and Yellow Transparent, sug- 

 gesting that gene A controls parasitism on the first two varieties, 

 and its allele, gene a, controls parasitism on the other two. This 

 situation only applies to the specific inbred stock of Venturia re- 

 ported on, for earlier papers showed that the same gene did not 

 control parasitism against these two varieties in all Venturia cul- 

 tures. These indications suggest that study of the genetics of this 

 fungus, especially the nutritional and vitamin deficiencies involved 

 in the pathogenic stocks, give great promise of solving many prob- 

 lems involved in the host -parasite relationship. 



GLOMERELLA 



Glomerella is an ascomycete similar to Neurospora, except 

 that it produces homothallic haploid spores. The fungus has been 

 studied quite thoroughly by Edgerton, Chilton, and Lucas, by Hut- 

 tig, and by Andes. They have all encountered the homothallic light 

 cultures which revert to the self-sterile dark stocks incapable of 

 producing large quantities of fertile perithecia from a single asco- 

 spore culture. The homothallic light stock ordinarily produces 



